1. Field of the Invention
The present invention relates to a liquid crystal light shutter capable of controlling the transmission of light, and, more particularly, it relates to a liquid crystal light shutter suitable for a recording device in which light writing is applied to the photosensitive drum.
2. Prior Art
A printer which employs the electrophotographic technique to achieve high speed printing has been developed and is used these days instead of the impact printers of the mechanical printing type. Laser beam, LED array, liquid crystal light shutter and the like have been proposed as the light writing means relative to the photosensitive matter. In the case of the printer which employs the liquid crystal light shutter, no mechanically-working section is needed to achieve the light writing, the printer can be produced as a solid-state device, and a desired light source which meets the sensitive wavelength of the photosensitive matter can be used.
Since this liquid crystal light shutter is needed to have a high speed response at the time of opening and closing its shutters, the so-called two-frequency frequency drive is employed as the method of driving the liquid crystal.
In the case of driving the liquid crystal light shutter according to the two-frequency drive, an electric field which includes a frequency component (which will be hereinafter referred to as f.sub.H) higher than a crossover frequency (at which the dielectric anisotropy of liquid crystal matter is inverted and which will be hereinafter referred to as f.sub.C) is applied to achieve dielectric dispersion and sourcefully provides homogeneous alignment when the liquid crystal light shutter which will be described later is turned off, while an electric field which includes a frequency component (which will be hereinafter referred to as f.sub.L) lower than f.sub.C is applied to force the direction of the electric field to homeotropic alignment when the liquid crystal light shutter which will be described later is turned on.
Optical characteristics of the liquid crystal light shutter of the Guest-Host type in which a dichroic dye is contained, will be described referring to Figs. 1A, 1B and FIGS. 3A, 3B.
FIGS. 1A and lB show a liquid crystal cell of the Guest-Host type (which will be hereinafter referred to as G.H type). The liquid crystal cell of the G.H type comprises solving the dichroic dye which serves as the Guest, into the liquid crystal which serves as the Host. Incident light 1, coming from a light source which has the same wavelength as the absorption wavelength of the dichroic dye, is linearly-polarized by a polarizing plate 2, which has a polarizing direction 2a, to become light 3, which enters into a liquid crystal cell 4. The liquid crystal cell 4 comprises liquid crystal molecules 5 and dichroic dye molecules 6, whose alignment directions are made changeable by the applied frequency f.sub.L or f.sub.H. Since the dichroic dye molecules 6 absorb more light in the direction of their long axis than in the direction of their short axis, the linearly-polarized light 3, which has entered into the liquid crystal cell 4 is absorbed, emitting no light 7 outside, in the case where the liquid crystal molecules 5 and dichroic dye molecules 6 are aligned as shown in FIG. 1A. Therefore, it is in a closed state when it is used as the liquid crystal light shutter.
When the liquid crystal molecules 5 and dichroic dye molecules 6 are so aligned, as shown in FIG. 1B, so as not to absorb the incident light 3, the light 7 is emitted outside. It is in the open state when it is used as the liquid crystal light shutter.
In order to expect a high speed response, however, the above-described liquid crystal light shutter is used while it is kept in a heated state and as it is reduced in viscosity. In addition, it is driven by a drive waveform in which many high frequency components of high voltage are contained, as represented by 31 in FIG. 7A, for example, at the time of its off-drive. The liquid crystal molecules 5 are not completely aligned parallel to the aligning-treatment direction of the liquid crystal cell 4 under these circumstances. Therefore, the dichroic dye molecules 6, also cause a shift 11, as shown in FIG. 3B, relative to the regular aligning direction 8 which is parallel to the aligning-treatment direction shown in FIG. 3A. Because of this shift 11, the linearlypolarized light is not completely absorbed by the dichroic dye molecules 6, thereby emitting a very small amount of leaked light outside. Numeral 10 in FIGS. 3A and 3B denotes the polarized direction of light 3, which has been linearly-polarized by the polarizing plate 2. As shown in FIG. 4, the polarized direction of light, which has been double-refracted inside the liquid crystal cell 4, is different from its originally-polarized direction because of the anisotropy of the refracting rate of the irregularly-oriented liquid crystal molecules 5. Therefore, a shift between the dichroic dye molecules and the polarized direction of double-refracted light becomes more remarkable, so that a composite consisting of leaked light components 12 and double-refracted, but not-absorbed components 13 becomes emitted light 14, thereby making the off-state of the above-described shutter incomplete.
When the liquid crystal light shutter repeats its on-off operation, this phenomenon is different between single off-operation and continuous off-operations. In short, the amount of light emitted is larger at the time of continuous off-operations than at the time of single off-operation, and its level becomes different depending upon temperature.